Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power output. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. Turbochargers typically include an intake air compressor rotationally coupled with an exhaust gas turbine, where the turbine provides thrust to the compressor by extracting energy from the exhaust flow.
The inventors herein have recognized a potential issue with turbocharged engines. Boosted engine operation at higher altitude regions may be sluggish. Specifically, the smaller turbocharged engines may lose power, particularly at low engine speeds such as engine idling speeds, due to the lower atmospheric air density. Air and corresponding power losses may also be incurred during transmission gear shifts. Further, the slower turbine response can lead to turbo lag and degraded boosted engine performance. In some engine systems, a wastegate coupled across the turbine can be used to increase engine air flow at idle to improve engine speed control. For example, the wastegate may be held closed to raise exhaust pressure upstream of the turbine. However, if the wastegate were used to increase engine air flow at idle conditions at the higher altitudes, the engine may demand to be operated with boost substantially all the time, including at idling conditions. As such, engine fuel economy may be reduced and noise, vibration, and harshness (NVH) may increase.
In view of the above, the inventors have recognized that electric compressor assist devices may be advantageously applied to assist in boosted engine idle speed control at higher altitudes. For example, a battery-driven electric motor coupled to the turbocharger compressor can be applied to provide sufficient boost to hold engine idling speeds without unnecessarily increasing engine speed to generate more torque. In one example, the above issue may be addressed by a method for a boosted engine comprising an electric motor powered by a battery bank to assist a compressor at higher altitudes.
As an example, an engine system may be configured with a turbocharger having a compressor driven by a turbine, the compressor further receiving, selectively, electric compressor assist (e.g., applying positive or negative torque to a turbocharger shaft or turbocharger components to accelerate or decelerate a compressor) from a battery-powered motor. In one example, the engine system may be coupled in a hybrid electric vehicle and the motor may be driven by a vehicle battery. During vehicle operation at higher altitude conditions, a ratio of assist provided to the compressor from the electric motor relative to assist provided to the turbine from an exhaust wastegate, may be varied based on a state of charge (SOC) of the vehicle battery to increase engine air intake pressure and improve engine idle speed control. For example, at higher battery SOC, more electric compressor assist may be provided while the wastegate may be left more open. In comparison, at lower battery SOC, less electric compressor assist may be provided while the wastegate may be left more closed. Further still, the electric motor may only be operated intermittently during engine idle speed control at higher altitudes, such as when required to raise engine speed to the idle speed, and may thereafter be deactivated. This allows the compressor assist to be advantageously used to maintain engine idling speeds at the higher altitude even before a tip-in event (e.g., increase in accelerator pedal position) occurs. Then, at the tip-in event, the electric compressor assist may be further increased as the boost demand increases to reduce turbo lag. The amount of electric motor assist provided to the compressor at higher altitudes may be similarly varied during a transmission gear shift to enable engine boost control during the gear shift. For example, less electric compressor assist may be provided during a transmission downshift at higher altitudes as compared to during a transmission upshift at higher altitudes. Compressor assist from the turbine provided via the wastegate may be correspondingly reduced. In each case, the amount of electric compressor assist provided may be adjusted (e.g., increased) as the altitude at which the vehicle is operating increases above a threshold to compensate for the corresponding drop in atmospheric air density.
In this way, boosted engine performance at higher altitudes can be improved without degrading fuel economy or raising NVH concerns. By increasing a ratio of electric compressor assist provided to a turbocharger compressor at higher altitudes, improved engine speed control may be achieved while an engine is idling and before a demand for boost pressure is received (e.g., such as due to a tip-in). Similarly, the electric compressor assist may be used to improve engine speed control during a transmission upshifting and downshift at the higher altitudes. By improving engine idling control, sluggish engine performance at higher altitudes may be averted. By coordinating wastegate adjustments with the electric compressor assist, usage of system battery charge during the providing of the electric compressor assist can be reduced, thereby improving overall hybrid vehicle performance.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. Thus, it should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.